First of all I need to know what truck this is on, what year, etc... Are you running a mechanical or electric fuel pump?
I went out and looked at both trucks on my place with 20r carbs and neither of them have that second fitting. The one with the hex head is in fact your supply, you are correct. On all mine the return line is from the return on the mechanical fuel pump and the return never reaches the carb at all.
If this is a '79 with an electric fuel pump that may be how they do the return. I haven't seen one like that myself.
That second fitting does however look like a thing that vents vapors from the float bowl on a 22r carb and just goes to a charcoal canister.
